Electric field effects on the carbon-13 chemical shifts of model compounds

Seidman, Kurt; Maciel, Gary E.

doi:10.1021/ja00452a009

Cited by 31 publications

(13 citation statements)

References 2 publications

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“…values are, and the correlation with E, is slightly better than that for cr, (Table 6,(24)(25)(26). Correlation of S(C-1) with E, is also improved slightly; the 'corrected' S(C-2) values cover a narrower range, and the correlation is poor.…”

Section: Discussionmentioning

confidence: 89%

Remote polar substituent effects on ¹³C NMR spectra of olefinic and aromatic carbons

Hill

Guenther

1981

Org. Magn. Reson.

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Chemical shifts of the olefinic carbons of o-substituted 1-pentenes and 1-butenes and the aromatic carbons of phenylethanes and phenylpropanes have been measured. Correlations with various substituent constants, electronegativities, NMR data from related chemical systems and calculated electrical fields were evaluated. Although the pattern of results is quite consistent with expectations for a field-induced shift of =-electron density, simple correlations of high statistical precision are not found. The general problem of correlating substituent effects with reactivity and NMR shifts is discussed.In the course of other studies, we observed that the chemical shifts of the olefinic carbons of 5-bromo-lpentene differ from those of its Grignard reagent by several parts per million. Since the position of substitution is 8 and E to the affected carbons, the substituent effects appeared to be rather larger than expected. We therefore studied a number of related compounds. We have concluded that the magnitude of the substituent effects is indeed 'normal', resulting from .rr-electron polarization by the polar substituent groups, and does not represent a specific interaction unique to these particular substituents. A number of other recent studies of NMR substituent effects also support the concept of bond electron polarization by polar substituents.'-'6 In this paper we report the chemical shifts of the olefinic carbons of a number of w-substituted 1-butenes and 1-pentenes (1, n = 2, 3), the aromatic carbons of the phenyl analogs (2, n = 2, 3) and olefinic carbons of some related cyclic compounds, and our attempts to correlate these shifts with measures of polar substituent effects. RESULTSIn Tables 1 and 2, chemical shifts are reported for series 1 and 2, respectively. Spectra were run mostly at concentrations of 10-15% by volume in diethyl ether. Shifts for some compounds were concentrationdependent at higher concentrations, but little change appeared to result from further dilution. Omitted from correlations because of poor fit to plot of 6(C-1) vs 6(C-2).

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Section: Discussionmentioning

confidence: 89%

Remote polar substituent effects on ¹³C NMR spectra of olefinic and aromatic carbons

Hill

Guenther

1981

Org. Magn. Reson.

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“…This is 3-fold greater than the corresponding effects at other ring carbons (London,198 1) or for C, of leucine (Tran-Dinh et al, 1974). Major changes in chemical shift at carbons involved in unsaturated bonds have been predicted by the calculations of Batchelor (1975), Seidman & Maciel (1977, and specifically for amino acids by Horsley & Sternlicht (1968).…”

Section: 72mentioning

confidence: 99%

Nuclear magnetic resonance study of dihydrofolate reductase labeled with [.gamma.-13C]tryptophan

Groff¹,

London²,

Cocco³

et al. 1981

Biochemistry

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Dihydrofolate reductase isozyme 2 of Streptococcus faecium has been labeled with 13C in the C gamma position of tryptophan residues by growing the organism on a defined medium containing L-[gamma-13C]tryptophan (90% 13C). The 13C nuclear magnetic resonance (NMR) spectrum of the enzyme shows four well-resolved resonances which have nuclear Overhauser enhancements of 1.1-1.3. Values of T1 (spin-lattice relaxation time) and T2 (spin-spin relaxation time) are significantly less than predicted for an isotropically rotating, rigid sphere with no intermolecular dipole-dipole interactions. Three of the resonances have chemical shifts downfield from the 13C resonance of urea-denatured enzyme by amounts up to 1.43 ppm. The chemical shift of resonance 4 in the spectrum is 4.0 ppm upfield from Trp C gamma of urea-denatured enzyme. This large upfield shift is attributed to electric field effects generated by polar side chains. The two more upfield peaks both provide evidence that the corresponding tryptophan residues, WC and WD, each undergo chemical exchange between alternative microenvironments. In the case of WC, which gives a resonance with two components, exchange is slow (ve, exchange rate much less than 55 s-1), and the relative populations of the two stable states are in the ratio 2:3. WD is apparently in intermediate to fast exchange on the NMR time scale. With a two-state model, ve increases from approximately 90 to 150 s-1 as the temperature is increased from 5 to 25 degrees C. This increases in temperature is also accompanied by an increase in the fractional population of the minor downfield state(s), from about 0.062 at 5 degrees C to 0.24 at 25 degrees C. However, the data may also be interpreted as a temperature-dependent equilibrium between a continuum of many states. WD is tentatively identified with Trp-22 since comparison of the sequences of Lactobacillus casei dihydrofolate reductase and S. faecium dihydrofolate reductase and inspection of the crystal structure of the L. casei enzyme indicate that Trp-6, Trp-115, and Trp-160 are probably all involved in regions of beta sheet whereas Trp-22 is in a loop joining beta A to alpha B. Earlier crystallographic evidence for the Escherichia coli reductase suggests that in the methotrexate complex with this enzyme the corresponding loop has a good deal of flexibility. It is probable that in the uncomplexed S. faecium reductase the motion of this loop is the major mechanism for the exchange process involving Trp-22. The upfield chemical shift of resonance 4 is attributed to electric field effects on C gamma of Trp-22 produced by the carboxylate groups of Asp-27 and Asp-9. On the basis of the small difference between the chemical shift of resonance 3 and that of tryptophan C gamma in urea-denatured reductase, it is suggested that WC may be identified with Trp-6.

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“…From the 13C n.m.r. The validity of Apr values obtained for different peptides is based on the assumption that the observed upfield shift for Cp reflects a ''y effect" mechanism (Seidman & Maciel, 1977) or an electric field effect due to the carbonyl oxygen (Batchelor, 1975); in either case, the dihedral angle will be the only relevant parameter and the results should be nearly independent of the particular residue following the proline. 3).…”

Section: Concentration Dependence Of the Acpronhme Shifts In Cdc13mentioning

confidence: 99%

“…A&Y = 6.0 p.pm. Based on recent theoretical evaluations of the reffect (Seidman & Maciel, 1977), the upfield shift of Cp should depend on the orientation of the Cp protons relative to the oxygen atom. However, the values for f will be most valid for peptides possessing a relatively large amount of y-turn structure.…”

Section: Concentration Dependence Of the Acpronhme Shifts In Cdc13mentioning

confidence: 99%

Quantitative Evaluation of Γ‐turn Conformation in Proline‐containing Peptides Using13 C N.M.R.

London

1979

International Journal of Peptide and Protein Research

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The dependence o f the 13C shift difference o f proline carbons Cp and C, on the dihedral angle $ has been studied using the model peptide acetyl-D-proline Nmethylamide. The shift difference Ap, is shown to be correlated with the percent cis isomer about the acetylproline bond, both factors depending strongly on the degree of intermolecular hydrogen bonding. Both the fraction of trans peptide bond and the fractional y-turn conformation increase as the sample concentration is decreased in CDC13. A h values have been used to evaluate the fractional 7-turn probabilities in a number o f cyclic and linear peptides including thyrotropin releasing factor and bradykinin. Using this parameter, it is concluded that in bradykinin the y-turn probability is low in D 2 0 and not strongly ternperature dependent. In contrast, studies o f a model peptide f o r the portion of bradykinin believed to adopt a y-turn conformation are consistent with an increased y-turn probability in less polar solvents. Data for X-Pro-Y peptides ( Y = imino acid) indicate significantly reduced values of A h , and this appears to be a useful basis for assigning the Pro Cp resonances corresponding to this sequence.

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Electric field effects on the carbon-13 chemical shifts of model compounds

Cited by 31 publications

References 2 publications

Remote polar substituent effects on ¹³C NMR spectra of olefinic and aromatic carbons

Remote polar substituent effects on ¹³C NMR spectra of olefinic and aromatic carbons

Nuclear magnetic resonance study of dihydrofolate reductase labeled with [.gamma.-13C]tryptophan

Quantitative Evaluation of Γ‐turn Conformation in Proline‐containing Peptides Using13 C N.M.R.

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Electric field effects on the carbon-13 chemical shifts of model compounds

Cited by 31 publications

References 2 publications

Remote polar substituent effects on 13C NMR spectra of olefinic and aromatic carbons

Remote polar substituent effects on 13C NMR spectra of olefinic and aromatic carbons

Nuclear magnetic resonance study of dihydrofolate reductase labeled with [.gamma.-13C]tryptophan

Quantitative Evaluation of Γ‐turn Conformation in Proline‐containing Peptides Using13 C N.M.R.

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Remote polar substituent effects on ¹³C NMR spectra of olefinic and aromatic carbons

Remote polar substituent effects on ¹³C NMR spectra of olefinic and aromatic carbons